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Have a look at the definition of the messages. The file UInt8MultiArray contains a comment:

# Please look at the MultiArrayLayout message definition for
# documentation on all multiarrays.

The file MultiArrayLayout.msg contains the following:

# The multiarray declares a generic multi-dimensional array of a
# particular data type.  Dimensions are ordered from outer most
# to inner most.

MultiArrayDimension[] dim # Array of dimension properties
uint32 data_offset        # padding bytes at front of data

# Accessors should ALWAYS be written in terms of dimension stride
# and specified outer-most dimension first.
# 
# multiarray(i,j,k) = data[data_offset + dim_stride[1]*i + dim_stride[2]*j + k]
#
# A standard, 3-channel 640x480 image with interleaved color channels
# would be specified as:
#
# dim[0].label  = "height"
# dim[0].size   = 480
# dim[0].stride = 3*640*480 = 921600  (note dim[0] stride is just size of image)
# dim[1].label  = "width"
# dim[1].size   = 640
# dim[1].stride = 3*640 = 1920
# dim[2].label  = "channel"
# dim[2].size   = 3
# dim[2].stride = 3
#
# multiarray(i,j,k) refers to the ith row, jth column, and kth channel.